splice.nix: make pkgs splicedPackages when required

This will make `pkgs` used in `callPackage`, and `pkgsCross.X.pkgs` have
packages with `__spliced`.

https://www.github.com/NixOS/nixpkgs/blob/3029741718f4c765fbc5ebf76bea3d6c8ff15fe5/pkgs/development/interpreters/python/passthrufun.nix#L37

https://www.github.com/NixOS/nixpkgs/blob/d2bd9a39dec88eddd5c192abee69939e67f43d12/pkgs/top-level/python-packages.nix#L10720

```
nix-repl> pkgsCross.aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced
error:
       … while evaluating the attribute 'aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced'
         at /home/artturin/nixgits/my-nixpkgs/.worktree/1/pkgs/development/python-modules/protobuf/4.nix:119:13:
          118|   passthru = {
          119|     inherit protobuf;
             |             ^
          120|   };

       error: attribute '__spliced' missing
       at «string»:1:1:
            1| pkgsCross.aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced
             | ^
```

to

```
nix-repl> pkgsCross.aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced
{
  buildBuild = «derivation /nix/store/s7da5mfvx4h1n86j78knaj9cprglxqz6-protobuf-25.4.drv»;
  buildHost = «derivation /nix/store/s7da5mfvx4h1n86j78knaj9cprglxqz6-protobuf-25.4.drv»;
  buildTarget = «repeated»;
  hostHost = «derivation /nix/store/mszvybzs4zxh43awyrjnybsfcb265n9r-protobuf-aarch64-unknown-linux-gnu-25.4.drv»;
  hostTarget = «repeated»;
}
```

pkgs/top-level/splice.nix

@@ -151,6 +151,8 @@ in
 
   newScope = extra: lib.callPackageWith (pkgsForCall // extra);
 
+  pkgs = if actuallySplice then splicedPackages // { recurseForDerivations = false; } else pkgs;
+
   # prefill 2 fields of the function for convenience
   makeScopeWithSplicing = lib.makeScopeWithSplicing splicePackages pkgs.newScope;
   makeScopeWithSplicing' = lib.makeScopeWithSplicing' { inherit splicePackages; inherit (pkgs) newScope; };